`enum.Flag.__contains__` changed behavior since python 3.12

[timrid]

Bug report

Bug description:

I noticed some strange behavior with enum.Flag an the __contains__ method in Python 3.12/3.13, as shown in the following examples.

Problem 1: Behavior changes at runtime
In the following code snippet the first print statement returns False, which is expected, since 3 is not a member of Weekday. However, the second print statement returns True, which is unexpected, since 3 is still not a member of Weekday.

import enum

class Weekday(enum.Flag):
    MONDAY = 1
    TUESDAY = 2
    WEDNESDAY = 4
    THURSDAY = 8
    FRIDAY = 16
    SATURDAY = 32
    SUNDAY = 64

print(f"{3 in Weekday}") # => False (expected)
_ = Weekday.MONDAY | Weekday.TUESDAY
print(f"{3 in Weekday}") # => True (not expected)

Problem 2: Behavior is not comparable to Python 3.11
Since Python 3.12 the behavior of Enum.__contains__ has changed, so that it is possible to compare not only with an enum-member, but also with non-enum-members (see here or here). So with Python 3.11 the code above will raise an TypeError: unsupported operand type(s) for 'in': 'int' and 'EnumType'. There you have to change the code to the following. But this in turn always produces unexpected behavior in Python 3.12/3.13.

import enum

class Weekday(enum.Flag):
    MONDAY = 1
    TUESDAY = 2
    WEDNESDAY = 4
    THURSDAY = 8
    FRIDAY = 16
    SATURDAY = 32
    SUNDAY = 64

print(f"{Weekday(3) in Weekday}") # => Python 3.11: False (expected)    Python 3.12/3.13: True (not expected)
_ = Weekday.MONDAY | Weekday.TUESDAY
print(f"{Weekday(3) in Weekday}") # => Python 3.11: False (expected)    Python 3.12/3.13: True (not expected)

Conclusion
I would have expected that in all cases the result is False, but since Python 3.12 it gets very strange...

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

Windows

Linked PRs

• gh-131045: [Enum] fix flag containment checks when using values #131053
• [3.13] gh-131045: [Enum] fix flag containment checks when using values (GH-131053) #131167
• [3.12] gh-131045: [Enum] fix flag containment checks when using values (GH-131053) #131232

[picnixz]

1 | 2 becomes 3 and I think your surprise comes from the fact that a new member is automatically created. I think you can avoid this starting from 3.12 by specifying the boundary (namely a new member is not automatically created). I haven't checked but this should also have been documented somehow maybe indirectly.

I believe that the reason is because the result of the OR of two enum flag members should remain an enum flag member by default (3.12+) and thus we create that additional member on the fly.

With a different boundary the result would be a pure int and not an enum member. To check if this diagnosis is correct, print the type of the _ in your code.

I can check on Wednesday as I'm unavailable until then (and I'm only answering from memory)

[tomasr8]

This behaviour is documented for enum.IntFlag:

If a Flag operation is performed with an IntFlag member and:

• the result is a valid IntFlag: an IntFlag is returned
• the result is not a valid IntFlag: the result depends on the FlagBoundary setting

Perhaps we should mention this in enum.Flag as well?

[ethanfurman]

The intent is that __contains__ will return True if either a member or a member value exists. In other words, if Weekday(x) would return a valid member, then x in Weekday should return True. The issue you found is that pseudo-members are not created until needed, but are then added to the Enum so that Weekday(3) is Weekday(3) is True (i.e. pseudo-members are also singletons).

One solution is to have __contains__ attempt a value -> member conversion so any pseudo-members are created.